Process and apparatus for carrying out dewatering of suspensions is disclosed in U.S. Pat. No. 4,168,222, issued Sept. 18, 1979, U.S. Pat. No. 4,207,158, issued June 10, 1980 and copending application Ser. No. 109,931, filed Jan. 4, 1980, which matured to U.S. Pat. No. 4,303,492 on Dec. 1, 1981. In these earlier disclosures, self-contained hollow electrode assemblies are provided which are normally submerged in the suspension, but bodily removable therefrom for inspection or other purposes. These hollow electrodes comprise two types of wall surfaces: ion-pervious walls for electrode assemblies of one polarity and liquid-pervious walls for electrode assemblies of the opposite polarity. The wall surfaces of the electrode assemblies comprise chemically and electrically neutral filter media or permeable porous membrane backed by a supporting grid, thus presenting a planar electrode surface.
In operation, with electrode assemblies of both types immersed in the suspension, a source of vacuum is connected to the interior of the electrode assemblies having liquid-pervious walls to provide a controllable pressure differential thereby producing a flow of carrier liquid through the filtration surfaces, while the solids migrate in the opposite direction, under the influence of the electric field, to deposit as cake upon the electrode assemblies having ion-pervious walls. Filtrate liquid, that is, carrier liquid freed of solids, is withdrawn or pumped from the interior of the liquid-filled hollow electrode structure at a controllable rate.
As indicated, cake deposition occurs on the hollow electrode assemblies having ion-pervious surfaces; these electrode assemblies being filled with an electrolyte and having an electrode element therein immersed in the electrolyte, isolated from direct contact with the suspension. The electrolyte is specially selected for high conductivity and compatibility with the electrode element. By compatability is meant the realitively non-corrosive character of the electrolyte under the conditions that ordinarily prevail within the hollow electrode assembly. Since decomposition or evolution products and heat are generated at the electrode element within the hollow ion-pervious electrode assembly, provision is made for a flow of electrolyte into and through the electrode chamber so that such foreign products and heat are swept out of the chamber and a relatively constant predetermined electrolyte composition is maintained.
The ion-pervious wall of the electrode assembly in these prior art structures comprises a chemically and electrically neutral filter media or permeable porous membrane which, if film-like in nature or otherwise requiring support, may be backed by a chemically and electrically neutral grid so that a planar electrode filter surface is presented to the slurry being treated. Since the cake will form on this electrode during electrofiltration and must be removed by contact with doctoring blades, a friction cage or spacing means may be provided to protect the filter medium from direct contact with the doctoring blades. The friction cage comprises a thin, open lattice or screenwork of relatively hard material covering the filter medium for contact with the doctoring blades while spacing means may comprise strips of plastic materials, such as Delrin acetal resin arranged in a frame-like configuration and of a thickness sufficient to prevent contact between the doctor blade and the filter medium. For the purpose of cake recovery, the electrode assembly may be raised to a position of emergence from the suspension, with the layer of collected solids or cake layer adhering thereto. Since the electrolyte remains within the electrode assembly in the raised position, a vacuum is applied internally of the electrode to reduce the pressure on the filter media and so prevent rupture of the filter media. When the electrode assembly is immersed in operation, the vacuum applied internally serves to remove gaseous products, such as chlorine or carbon dioxide evolved at the electrode element.
The ion-pervious electrode structures of the prior art, applied in the field of clay dewatering, exhibit certain operating problems. Since the clay particles in the feed material are of colloidal particle size, a significant amount of such clay particles pass through the filter medium. This clay accumulates in the chamber of the electrode assembly, contaminates the electrolyte circulating through the clay and restricts and blocks flow of electrolyte through the electrode chamber. Ultimately, the electrode assembly must be taken out of operation, disassembled, cleaned and reassembled, a time-consuming and expensive procedure. Further, it has been found that, electrolyte is being pumped from the chamber of the electrode assembly into the bath. This infusion of material into the bath materially and adversely affects the properties of the dewatered product.
In copending patent application Ser. No. 222,057, filed Jan. 2, 1981, there is disclosed a means for overcoming the above difficulties which involves the use of an ion-exchange resin membrane as the ion-pervious wall of the electrode assembly. In particular, the copending application points out that commercially available cation exchange (perfluorosulfonic acid) membranes sold under the trademark NAFION by E. I. duPont de Nemours Company function satisfactorily as the ion-pervious wall of the electrode, essentially eliminating mass flow of clay solids into the electrode assembly and of anolyte into the bath, while permitting passage of cations.
While these commercially available ion exchange materials represent a substantial improvement over the prior art ion-pervious wall materials, they do have significant disadvantages for practical commercialization. They are extremely expensive, costing perhaps $30 or more per square foot. Their dimensions may change 10-20% between a dry and wet state. They cannot be adhesively bonded or glued, making fabrication into leak-proof shapes with good mechanical properties difficult, if not impossible.
Accordingly, there exists a real need for an improved solid electrolyte as an ion-pervious electrode structure for electrofilter apparatus.